The present invention relates to an image processing system that may be employed in association with an optical detection and acquisition system, such as a camera, in order to improve the quality of images acquired by the detection and acquisition system.
In the design of optical detection and acquisition systems, a significant problem has been the susceptibility of such systems to aliasing distortions or undersampling artifacts. This effect occurs when a detection system acquires an image having a visual pattern containing high spatial frequency components as, compared to the spatial sampling density of the photoreceptor array employed in the detection system. Thus, if a camera having a solid state array of photosensors is directed at a pattern of alternating dark and light fine stripes, a familiar moire pattern having wavy or flickering lines can distort the resolved image.
It is well known that aliasing distortion can be eliminated by removing spatial frequency components in the image spectrum that exceed half of the spatial sampling rate. For this purpose, low pass “blur” filters based on birefringent materials have been developed that split the light being received at any one input so that the light received by any one photoreceptor represents an average of the light received at two or more adjacent inputs. Typical constructions of passive filters of this type are shown in Greivenkamp, Jr., U.S. Pat. No. 4,575,193, Sato et al., U.S. Pat. No. 4,626,897, and Weldy et. al., U.S. Pat. No. 4,663,661. In Asaida, U.S. Pat. No. 4,761,682, a cascaded design is shown wherein three serially arranged crystal plates (birefringent crystals) are used to project a pattern on the photosensor of eight separate rays for each single input ray. A substantial difficulty with such designs, however, is that the blur filter significantly limits the capacity of the detection system to produce sharp images, i.e., anti-aliasing and retaining sharpness are competing goals.
In order to achieve one or both goals of preventing aliasing distortion and enhancing sharpness, dynamically controllable image processors have been developed. Nohda et al., U.S. Pat. No. 5,369,266, disclose solid-state image pickup devices for imaging an object using a CCD image sensor. The device, as shown in FIG. 1, includes a birefringent plate that splits the incident beam into two plane polarized rays of mutually perpendicular polarizations that are spatially displaced from each other. The rays are then incident on an electronically controllable liquid crystal plate that selectively rotates the state of the incident ray or passes the state of the incident ray unchanged. A polarizer then blocks one or the other of the rays dependant on its state so that the output beam alternates between a first and a second linearly displaced position. Thus, the input images can be shifted back and forth between the photoreceptors to allow interpixel sampling of the input image for enhanced sharpness. However, the embodiment shown in FIG. 1, is a single stage device only functional for enhancing the sharpness of an image in a single linear direction.
Nohda et al. disclose in FIG. 2 a further embodiment of a device that includes a dual pair of electronically controllable birefringent crystals is used having their respective optic axes arranged so that the light recombines in the device after splitting, while still providing an output beam shiftable between two linearly displaced positions. Unfortunately, the crystals need to have perfectly matched characteristics and the dual electronic controls need control signals that exactly correspond electrically for the device to operate properly.
Other references that disclose image processors for shifting an input beam between a pair of linearly spaced output positions are disclosed by Hasegawa et al., U.S. Pat. No. 4,882,619; Nishioka et al., U.S. Pat. No. 5,091,795; and Tatsumi, U.S. Pat. No. 5,764,287. In particular, Hawegawa et al. show an electronically switchable liquid crystal and prism assembly for shifting the input beam for interpixel resolution enhancement.
Niskioka et al., in particular, disclose in FIGS. 12A and 12B an image processor for linearly shifting a beam between two output positions wherein the beam is first divided by a controllable birefringent plate and then recombined by a second controllable birefringent plate to divert the beam to one of the output positions. In this respect, Niskioka et al. structure is like that shown in Nohda et al., however, in the Niskioka et al. device the driving voltage is changed at a speed twice the readout period of the CCD sensor. The effect is to cause low pass filtering for reducing aliasing rather than to enhance sharpness. In any event, in this respect it shares Nohda et al.'s disadvantage of requiring matched electronically controlled devices.
Tatsumi in particular discloses in a first embodiment yet another image processor for shifting one input beam between a pair of output positions in order to achieve interpixel sampling for enhanced sharpness. The incident light beam is passed through a polarizer and then through an electronically controllable liquid crystal element that transmits a plane polarized beam having one or another rotation state. The light then proceeds to a birefringent crystal where the light beam is directed to one or another of the output positions depending on its state. In a second embodiment, Tatsumi discloses a phase plate and second birefringent crystal that are added to the assembly of the first embodiment to obtain a pair of simultaneous output beams thereby providing the blurring effect. Each output beam is linearly shiftable, in a single direction as before, using the electronically controllable liquid crystal element so as to provide interpixel sampling thereby achieving enhanced sharpness compatibly with the blurring anti-aliasing effect of the simultaneous beams. Unfortunately, this dual birefringent crystal device only enhances sharpness along a single dimension. Moreover, the simultaneous beams limit the capacity of the device for sharply resolving finely detailed images.
Another patent that is directed toward compatibly achieving simultaneous sharpness enhancement and aliasing reduction is Okada et al., U.S. Pat. No. 5,834,761. Okada et al. accomplish sharpness enhancement by mechanically tilting a double refraction plate to produce a shifting beam. At the same time a pair of double refracting cells following the refraction plate are mechanically rotatable relative to each other to split the shifted beam into simultaneous output beams to accomplish the blurring effect for aliasing reduction. The moving parts decrease reliability, require excessive space to operate, are cumbersome, are bulky, tend to get out of alignment, and are not reliable. Moreover, the simultaneous beams limit the capacity of the device for sharply resolving finely detailed images.
Referring again to Niskioka et al., a two stage image processor is described, (see generally FIGS. 6–8), wherein each stage includes a birefringent crystal followed by an electronically controllable liquid crystal element, succeeded by another birefringent crystal. A quarter wave plate is interposed between the two stages. Each light ray incident on the assembly is split into four simultaneous output beams for blurring, where the output beams are each shiftable in a two-dimensional pattern in reverse directions from the other. Unfortunately, each output beam is derived from light entering multiple input locations. This results in a loss of information and a resulting reduction in sharpness enhancement capacity.
What is desired, therefore, is an image processing system that is suitable to remove undesirable alias distortion and/or perform resolution enhancement.